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Tweak ligature component matching for ligature formation

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If two marks want to ligate and they belong to different components of the
same ligature glyph, and said ligature glyph is to be ignored according to
mark-filtering rules, then allow.

Example Burmese senquence:

  U+1004,U+103A,U+1039,U+101B,U+103D,U+102D

Test font provided by Norbert Lindenberg.

Fixes https://github.com/behdad/harfbuzz/issues/545
pull/553/head
Behdad Esfahbod 7 years ago
parent 71c0a1429d
commit 8b2c94c43f
3 changed files with 51 additions and 9 deletions
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  1. 59
      src/hb-ot-layout-gsubgpos-private.hh
  2. BIN
      test/shaping/fonts/sha1sum/a6c76d1bafde4a0b1026ebcc932d2e5c6fd02442.ttf
  3. 1
      test/shaping/tests/ligature-id.tests

59
src/hb-ot-layout-gsubgpos-private.hh
Unescape View File

@ -374,6 +374,13 @@ struct hb_apply_context_t :
inline void reject (void) { num_items++; match_glyph_data--; }
inline matcher_t::may_skip_t
may_skip (const hb_apply_context_t *c,
const hb_glyph_info_t &info) const
{
return matcher.may_skip (c, info);
}
inline bool next (void)
{
assert (num_items > 0);
@ -736,11 +743,17 @@ static inline bool match_input (hb_apply_context_t *c,
* - Ligatures cannot be formed across glyphs attached to different components
* of previous ligatures. Eg. the sequence is LAM,SHADDA,LAM,FATHA,HEH, and
* LAM,LAM,HEH form a ligature, leaving SHADDA,FATHA next to eachother.
* However, it would be wrong to ligate that SHADDA,FATHA sequence.o
* There is an exception to this: If a ligature tries ligating with marks that
* belong to it itself, go ahead, assuming that the font designer knows what
* they are doing (otherwise it can break Indic stuff when a matra wants to
* ligate with a conjunct...)
* However, it would be wrong to ligate that SHADDA,FATHA sequence.
* There are a couple of exceptions to this:
*
* o If a ligature tries ligating with marks that belong to it itself, go ahead,
* assuming that the font designer knows what they are doing (otherwise it can
* break Indic stuff when a matra wants to ligate with a conjunct,
*
* o If two marks want to ligate and they belong to different components of the
* same ligature glyph, and said ligature glyph is to be ignored according to
* mark-filtering rules, then allow.
* https://github.com/behdad/harfbuzz/issues/545
*/
bool is_mark_ligature = _hb_glyph_info_is_mark (&buffer->cur());
@ -761,13 +774,41 @@ static inline bool match_input (hb_apply_context_t *c,
unsigned int this_lig_id = _hb_glyph_info_get_lig_id (&buffer->info[skippy_iter.idx]);
unsigned int this_lig_comp = _hb_glyph_info_get_lig_comp (&buffer->info[skippy_iter.idx]);
if (first_lig_id && first_lig_comp) {
if (first_lig_id && first_lig_comp)
{
/* If first component was attached to a previous ligature component,
* all subsequent components should be attached to the same ligature
* component, otherwise we shouldn't ligate them. */
* component, otherwise we shouldn't ligate them... */
if (first_lig_id != this_lig_id || first_lig_comp != this_lig_comp)
return_trace (false);
} else {
{
if (first_lig_id != this_lig_id && this_lig_id != 0)
return_trace (false);
/* ...unless, we are attached to a base ligature and that base
* ligature is ignorable. */
bool found = false;
const hb_glyph_info_t *out = buffer->out_info;
unsigned int j = buffer->out_len;
while (j && _hb_glyph_info_get_lig_id (&out[j - 1]) == first_lig_id)
{
if (_hb_glyph_info_get_lig_comp (&out[j - 1]) == 0)
{
j--;
found = true;
break;
}
j--;
}
if (!found)
return_trace (false);
if (skippy_iter.may_skip (c, out[j]) != hb_apply_context_t::matcher_t::SKIP_YES)
return_trace (false);
}
}
else
{
/* If first component was NOT attached to a previous ligature component,
* all subsequent components should also NOT be attached to any ligature
* component, unless they are attached to the first component itself! */

BIN
test/shaping/fonts/sha1sum/a6c76d1bafde4a0b1026ebcc932d2e5c6fd02442.ttf
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Binary file not shown.

1
test/shaping/tests/ligature-id.tests
Unescape View File

@ -33,3 +33,4 @@ fonts/sha1sum/1c2fb74c1b2aa173262734c1f616148f1648cfd6.ttf::U+0995,U+09CD,U+0995
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fonts/sha1sum/a6c76d1bafde4a0b1026ebcc932d2e5c6fd02442.ttf::U+1004,U+103A,U+1039,U+101B,U+103D,U+102D:[uni101B103D=0+450|uni1004103A1039102D=0@-50,0+0]

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